I think I can see the argument he’s trying to make.If your caps combust while heating you're going too far.
Do they actually *combust*? I doubt that. Anyway, cleaning those kind of residues with a toothbrush and some toothpaste removes it entirely.
If you’re already riding the line then the cap does indeed get heated to combustion temp, as it gets to a slightly higher temp than the bowl which it is passing heat to.
I suppose the question is exactly the one you posed, do the substances on the cap “combust” and does this enter the air path? I’m like you, not so sure.
Either way we can all agree it’s way less damaging than full combustion and inhalation of the burning material